fix: add validation expected behavior

Author: fangting-alchemy

standard/ERCs/erc-6900.md

@@ -101,10 +101,10 @@ Each step is modular, supporting different implementations for each execution fu
 
 **Modules** **May** implement one of the following module types
 
-- `IValidationModule`
-- `IValidationHookModule`
-- `IExecutionModule`
-- `IExecutionHookModule`
+- `IValidationModule` to support validations for account.
+- `IValidationHookModule` to support hooks for validations.
+- `IExecutionModule` to support execution functions and their installations on account.
+- `IExecutionHookModule` to support pre & post execution hooks for execution functions.
 
 #### `IModuleManager.sol`
 
@@ -575,6 +575,12 @@ struct ModuleManifest {
 
 ### Expected behavior
 
+#### Validations and their installation /uninstallation
+
+An account can have more than one validation installed.
+An account can have the same validation module installed more than once.
+The entityId of a validation installed on an account MUST be unique.
+
 #### Responsibilties of `StandardExecutor` and `ModuleExecutor`
 
 `StandardExecutor` functions are used for open-ended calls to external addresses.